Low emissivity coatings are well known in the art. Typically, they include one or more layers of an infrared-reflective film and one or more layers of dielectric material. The infrared-reflective films, which typically are conductive metals such as silver, gold or copper, help reduce transmission of heat through the coating. The dielectric materials are used, for example, to reduce visible reflectance and to control other properties of the coatings, such as color and visible transmittance. Commonly used dielectric materials include oxides of zinc, tin, indium, bismuth, and titanium.
U.S. Pat. No. 4,859,532, issued to Oyama, et al. describes one simple low-emissivity coating. The patent describes a five-layered transparent coating having a zinc oxide layer formed directly on the substrate, which is typically a sheet of float glass or the like. A second silver layer is formed on the zinc oxide layer. A third zinc oxide layer is formed on the second silver layer. A fourth silver layer is formed on the third zinc oxide layer. Finally, a fifth zinc oxide layer is formed on the fourth silver layer. The thickness of the first and fifth zinc oxide layers is said to be 200-600 angstroms while the thickness of the third, middle zinc oxide layer is said to be 400-1200 angstroms. Both of the silver layers are 60-250 angstroms thick, with a range of 80-100 angstroms being said to be preferred. In coatings of this nature wherein the whole dielectric film region between the two silver layers is formed by a single zinc oxide layer, film defects may be more likely to extend through the entire thickness of this middle dielectric region.
It is often necessary to heat glass sheets to temperatures at or near the melting point of the glass to temper the glass or to enable the glass to be bent into desired shapes such as motor vehicle windshields. Coated glass articles often must be able to withstand high temperatures for periods of time up to several hours. Tempering, as is known, is particularly important for glass intended for use as automobile windows and particularly for use as automobile windshields. Upon breaking, tempered windshields desirably exhibit a break pattern in which they shatter into a great many small pieces rather than into large, dangerous sharp shards. Tempering temperatures on the order of 600° C. and above are required. Film stacks employing silver as an infrared-reflective film often cannot withstand such temperatures without some deterioration of the silver film. To avoid this problem, glass sheets can be heated and bent or tempered before they are coated, and later can be provided with the desired metal and metal oxide coatings. Particularly for bent glass articles, though, this procedure may produce non-uniform coatings and is costly.
One further problem encountered during tempering is the development of a haze within the film stack. It appears that this hazing is associated with the growth of crystals within the layers. When layers are initially deposited (e.g., via magnetron sputtering), they tend to have either a fairly amorphous microstructure or a rather small grain size. At the elevated temperatures associated with tempering, the crystals in these layers are believed to grow larger until they become large enough to have a direct effect on the light passing therethrough. This, it is surmised, causes haze in the coating when it is treated at elevated temperatures.
If the 5-layer Oyama et al. film stack were tempered at elevated temperatures, it is rather likely that the silver layers would be oxidized sufficiently to render the resulting coated glass article unsellable. Even if the film stack were modified to protect the silver layers, the tempering likely would reduce transmittance of the coating due to the development of a haze in the dielectric ZnO layers. The impact of this haze on the quality of the glass coating would depend on the tempering profile—longer times at elevated temperatures will further increase the hazing problem while shorter, cooler cycles will minimize (though not eliminate) the hazing problem.
The above description pertains primarily to efforts to produce glass structures useful as architectural glass or glass for automobile windows, in which the glass structures in use are not usually subjected to high temperatures after they have once been tempered or bent. Coated glass sheets may also find utility as windows for ovens of various types in which the windows are subjected to repeated heating and cooling cycles as the ovens are heated and cooled during normal usage. A good example of such usage is a self-cleaning kitchen oven in which the oven temperature may be repeatedly raised to cooking temperatures of 250° F. to 450° F. with frequent excursions to, e.g., 900° F. during cleaning cycles. An oven window of this type should be transparent to enable one to see through it into the oven. It should be highly reflective in the infrared range to retard heat loss from the oven and help keep the exterior of the oven from getting too hot. Further, it must be resistant to deterioration resulting from repeated temperature escalations while exposed to the conditions of humidity and chemical (food) oven conditions.